1. Field of the Invention
The present invention relates to highly hydrophobic porous membranes for use in air filters for removal of oil mists, porous membranes for deaerating liquids, water-vapor permeable hydrophobic porous membranes used in a low-temperature heat storage system based on differential aqueous solution concentration and dilution, membranes for use in a gas-liquid separation of highly basic solvents, such as diaminoethanol and monoaminoethanol used to absorb polar gases, such as CO.sub.2, SO.sub.2, and H.sub.2 S, and membrane distillation of highly corrosive liquids, and the like.
2. Description of the Related Art
Hydrophobic porous membranes are expected to find applications in the fields of gas-liquid separation by taking advantage of their hydrophobicity.
Hydrophobic porous membranes having excellent mechanical strength for use in these applications are known, which include porous membranes made of polyolefins, such as polyethylene and polypropylene, and more hydrophobic porous membranes which are made of fluorinated polymers, such as polytetrafluoroethylene and polyvinylidene fluoride.
Porous membranes made of polyolefins, such as polyethylene, which are known to be inexpensive and available in many varieties, are, however, insufficiently hydrophobic, for example, as water-vapor permeable hydrophobic porous membranes for use in a low-temperature heat storage system based on differential aqueous solution concentration and dilution, for which more hydrophobic porous membranes are desired. Porous membranes made of polytetrafluoroethylene or polyvinylidene fluoride are known to be more hydrophobic porous membranes. However, such membranes also find limited applications because their wettability with a solution having a surface tention of 28 mN/m or lower prevents their use in a gas-liquid separation of lower surface tension liquids; they exhibit lower mechanical strength compared to polyolefin porous membranes; and they are also expensive.
A hydrophobic porous membrane is known as a means to solve the above problems in which the hydrophobicity is enhanced by a process described in U.S. Pat. No. 4,954,256 which calls for exposing the pore surfaces of a porous membrane to gamma-rays, and graft polymerizing onto the porous membrane surface a highly hydrophobic fluorinated polymer, thereby, bonding it chemically to the membrane. However, this patent teaches nothing about crystallization of fluorinated alkyl side chains on the fluorinated polymer.
According to studies conducted by the present inventors, it was noted that the process which uses gamma rays for grafting is suitable for treating a readily graftable and relatively radiation-stable porous membrane with substrate, such as polyvinylidene fluoride, but grafting onto a porous membrane, particularly a polyolefin type tends to degrade the membrane substrate, resulting in decreased membrane mechanical strength. It was also noted that the process, when applied to a hollow membrane, results in a membrane with a different degree of hydrophobicity between the external and internal surfaces and it is difficult to make the internal surfaces hydrophobic.
Methods are also known to functionalize the surface of a porous membrane so as to improve hydrophilicity, heat resistance, or other properties, which involve holding a reactive monomer on the surface of a porous membrane and polymerizing it in situ as, for example, is disclosed in U.S. Pat. No. 4,695,592 and U.S. Pat. No. 4,961,853; however, these methods teach neither the use of reactive monomers that permit the crystallization of side chains nor any method for crystallizing the side chains. These methods, which call for holding a reactive monomer on the surface of a porous membrane and polymerizing it in situ, can hold a functional polymer uniformly reaching even the interior of the pores without any reduction in mechanical strength caused by degrading the porous membrane. The inventors studied these methods, whereby a highly hydrophobic fluorinated monomer was held on a polyethylene hollow fiber membrane and polymerized as a way as to improve hydrophobicity. However, it was noted that although the fluorinated polymer can be relatively uniformly applied to the pore surface, thereby improving hydrophobicity over the starting polyethylene hollow fiber membrane, the resultant hydrophobicity is not better than that of polytetrafluoroethylene. This may be because the fluorinated alkyl side chains of the fluorinated polymer are not crystallized.
For the foregoing reasons, there is a need for a polyolefin porous membrane which exhibits higher mechanical strength than a porous membrane based on a fluorinated polymer, such as polytetrafluoroethylene or polyvinylidene fluoride, which is a low cost substrate, and which has superior hydrophobicity compared to that of polytetrafluoroethylene membranes.